AppleScript: The Definitive Guide

Lifetime of Variables

The lifetime of a variable means just what you think it means—how long the variable lives.

A local variable is born when it first is assigned a value, and dies when the scope in which it was born stops executing. A variable that behaves this way is sometimes called an automatic variable, because it comes into existence and goes out of existence automatically. For example:

on myHandler(  )
        local x
        set x to 5
        display dialog x
end myHandler
myHandler(  ) -- 5
display dialog x -- error

Well, you already knew what would happen when that code runs. But what I’m saying now is something you can’t see, and I can’t quite prove, so you’ll just have to believe me: by the time we get to the last line of that example, the local x inside myHandler isn’t just unavailable, it’s gone. It came into existence as myHandler was executing, and it went out of existence when myHandler finished executing.

A top-level entity other than a local variable is persistent. This means that its name and its value survives the execution of the script. This becomes interesting and relevant if you execute the same script twice.

Here’s a simple example. Create this script in a script editor program and run it:

property x : 5
set x to x + 1
display dialog x -- 6

Now run it again, without doing anything else. Here’s what happens:

property x : 5
set x to x + 1
display dialog x -- 7

This amazing result is possible because AppleScript has a memory. (You did reread Section 4.2.4, didn’t you?) Your compiled script ...